Abstract: A valve body comprises two inlet ports for receiving fluid at respective different pressures, an outlet port for delivering said fluid, a valve member mounted for limited movement within said body, and biasing means for biasing said valve member to move to one limit of its movement, said valve member being operable to move in response to the difference in pressure at said first and second ports in response to said biasing means for causing the valve member to vary the respective contributions of fluid delivered to the outlet port from the inlet ports. The valve body can contain a further movable valve member which is operable for receiving fluid from isolating control means and, in response thereto, for moving to obstructions one of said inlet ports and for urging the first mentioned valve member to obstruct the other inlet port.